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A METEOROLOGICAL STUDY OF PARKS AND TIMBERED 

AREAS IN THE WESTERN YELLOW-PINE FORESTS 

OF ARIZONA AND NEW MEXICO 



BY 

G. A. PEARSON 

FOREST EXAMINER 



RepriDted from the Monthly Weather Review, Vol. No. 41, pages 1615 to 1629 



WASHINGTON ; GOTEBNMENT PBINTINO OFrlCB : 1914 



A METEOROLOGICAL STUDY OF PARKS AND TIMBERED 

AREAS IN THE WESTERN YELLOW-PINE FORESTS 

OF ARIZONA AND NEW MEXICO 



BY 

G. A. PEARSON 

FOREST EXAMINER 



T?e|irint('d from tlip ^ronthly Weather Review, Vol. No. 41, pages IfilS to 1629 



25318—14 1 






0. OF 0, 

f£8 21 1314 



A METEOROLOGICAL STUDY OF PARKS AND TIMBERED AREAS IN THE WESTERN YELLOW-PINE FORESTS OF 

ARIZONA AND NEW MEXICO. 



By G. A. Pearsox, Forest Examiner. 



OBJECT OF THE STUDY. 



CLIMATE. 



The object of the study, the results of which are pre- 
sented here, was to determine the infiuence of the forest 
cover upon climate locally in the Southwest, in so far 
as this influence might be of importance in the manage- 
ment of timberlands and the possible afforestation of 
parks and denuded areas. 

Since the bearing upon forestry rather than upon 
meteoi'ology is the primary consideration, comparatively 
little attention is given to purely meteorological prob- 
lems. The study, moreover, makes no pretense of in- 
vestigating the influence of the forest upon the general 
climate of the region. A study of the regional climatic 
influences would require more complete observations and 
an extended series of stations during a long period of 
years. Such observations have been made in Germany, 
Austria, Sweden, France, and other European countries, 
and have served as a foimdation for the present study, 
although, as the residts show, the relations foTUid to exist 
between forest and field in Europe are in many cases 
greatly modified under the vastly difl'erent conditions 
prevailing in Arizona and New Mexico. A comprehen- 
sive review of European investigations in forest meteor- 
ology is included in Forest Service Bulletin No. 7, 
Part II, and in the fuial report of the National Water- 
ways Commission, Senate Document 469. 

THE REGION. 
FOREST .\ND TOPOGRAPHY. 

The locality in which this study was made lies well 
within the Imiits of a practically continuous belt of forest, 
from 25 to 100 miles in width, and extending from north 
central Arizona in a southeasterly direction into south- 
western New Mexico, a distance of approxunately 350 
miles. In northern Arizona, where the study was made, 
the forest occupies an extensive plateau, known as the 
Colorado Plateau, which has a general elevation of from 
6,000 to .s,000 feet above sea level, with numerous peaks, 
the highest nearly 13,000 feet. Western yellow-pine 
(Pimis ponderosa) occupies the level mesas and lower 
slopes from approximately 6,500 to 8,500 feet, forming 
a belt from 10 to 30 miles ■wade, which extends almost 
the entire length of the timbered area and constitutes 
the largest pine forest on the continent. The mountain 
slopes above S.500 feet are covered mainly by Douglas 
fir (Psevdot.^uga taxifolia), white fir (Ahies concolor], 
limber pine (Pinvs flexilis). bristle-cone pine (P-hms 
ai'istata), and Engelmann spruce (Piaa fn<i<iriHimvl). 
Beginning at 6,500 feet and extending down to about 
5,000 feet is a woodland forest, composed mainly of two 
junipers {Jiniipcrvx pachypJiloca and Junipei'vs utaliensis), 
pihon pine {Pinus edulifi), together with several jiieces 
of oak. The woodland forms a belt from 10 to 25 miles 
WT.de, which niuy he. considered a transition zone between 
the timlier forest and the treeless area below. These 
zones of forest growth ai-e typical of the timl)er regions 
of Arizona and New Mexico; the altitudinnl limits, 
however, vary considerably with latitude and local con- 
ditions. 



The climate of this region, which is fairly typical of 
the higher altitudes of Arizona and New Mexico, is char- 
acterized by great daily ranges of tem])eratin'e and very 
marked seasonal variations in precipitation, atmospheric 
moisture, and wind movement. The percentage of sun- 
shine is very liigh. Owing to the excessive radiation 
characteristic of the high altitudes and dry climate in 
which the western yellow-pine forests occur, the change 
of temperature from day to night is usually very great, 
frequently amounting to more than 50° F. Practically 
all of the precipitation, which in the western yeUow-]3ine 
forest amounts to from 20 to 25 inches per year, falls 
during the summer and winter months, the former period 
extending approximately from July 1 to Se))tember 1, 
and the latter from December 1 to April 1. During the 
summer rainy season showers occur almost daily, and the 
relative humidity is usually high. During the winter 
months the ])reci])itation comes mainly in the form of 
snow, which, in the higher altitudes of the western yellow- 
])ine forest, accumulates to the depth of 3 feet or more. 
As a rule there is ])ractically no precipitation from about 
April 1 to July 1, and this period is characterized by ex- 
tremely rapid evaporation and fi-equent high winds. It 
is exceedingly trying on vegetation, especially forest 
seedUngs which have not become well establislied. A 
similar, though usually much less severe, jjeriod of 
drouth and wind extends from about September 1 to 
November 1. 

PARKS. 

The western j^ellow-pine forests of the region grow 
naturally in very ojien stands. These usually consist of 
small groups from 50 to 100 feet or more in \\-idth, of 
mature or nearly mature trees, separated by grouiis of 
younger trees, sometimes seedlings, sometimes saphngs, 
and sometimes poles. Frequently, however, there are 
openings bare of trees between the groups. These open- 
ings are usually several liundred yards wide, but occa- 
sionally they cover several square miles. Openings of 
the latter class are commonly known as "parks," or, if 
very large, they are sometimes called prairies. 

A number of theories have been advanced in explana- 
tion of the origin of the jiarks in this region. One is that 
they were once timbered but have subsequently been 
denuded by fires. A more plausible tlieory in the case of 
most parks is that the}'' are naturally treeless, owing to 
the presence of conditions unfavorable to tree growth. 

THE FORT VALLEY PARK. 

The park selected for this study lies about S miles 
northwest of the town of Flagstaff, on the Coconino Na- 
tional Forest, and is locally known as Fort Valley. It is 
approximately \\ miles wide by 2i miles long, the major 
diameter extending from northeast to southwest. (See 
fig. 1.) The to]iography is j^ractically level, and the av- 
erage elevation is ai)])roximately 7,250 feet. The soil is 
mainly a graveU}' alluvial loam underlain at depths rang- 
ing fi-om 1 to 3 feet by deposits of volcanic cinders. About 
two-thirds of the land is imder cultivation and the re- 



(3) 




Fir.. I. — Fort Valley Park, looking lowaid the Sail Francisco Mountains. The figures intlicalc the location of 

meteorological stations. 




Fig. 2.— Meteorological station No. 1, edge of forest. 




Fig. 3.— Meteorological station No. 2,?park. 




Fig. 4. — Meteorological station No. 3, forest. 



inaindor is covorcil l)y a fiiirly denso growth of <;ramma 
•rrass (lioufeloua oligofitachya). A luoderatcly heavy for- 
est of western yellow-pine, opened iiere and there by liglit 
ciittin<;s, surrounds llie park. Tlie linihor almost invari- 
ably oeevn's at a slii;:litly liij^iier elevation llian the adja- 
cent |)ark land. In some jilaces the tinil)ered area rises 
from tlie eds^e of the ])ark on a ver}' jjentlc slope, wlnle in 
others there is a rather abrupt rise of from 25 to 100 feet. 
Occasional narrow, sli<jhtly elevated tongues of timbered 
land extend out into tiie o]iening a distance of asmueli as 
one-fourlli mile, and in two instancessmall isolated patch- 
es of saplings occur on slight eminences w(dl out in the 
park. 

The ])ark ami tlie JTumediately suri'ounchng tind)ered 
areas present the ap])earance of a, partial basin. From 
3 to 4 miles to the north and oast of the park begin the 
steep slopes of the San Francisco Mountains which, 
in a liorizontal distance of 3 or 4 miles, rise from an eleva- 
tion of S, .")()() feet' to 12,340 feet, or approximately ,"),0()() 
feet above the level of the Fort ^'alley Paik. Minor 
peaks in tlie near A'ieinity are Wing Mountain, about 
2 miles nortliwest, and Crater Mountain, bordering on 
tiie soutli edge of the park, with elevations of S, .")()() and 
S.;;oi) feet, resp(>ctively. About G miles to tlie southeast, 
|iiactically connected with the San Francisco Mountains 
by a s(Ties of spurs and ridges, is Elden Mountain, 
with an elevation of i),2S0 feet. On the south and west 
sides the steep timbered slopes rise from 200 to 300 
feet above the park into a level mesa, wliieh here forms 
t he rim of tiie basin. 

METEOROLOGICAL OBSERVATION.S. 

Th(! meteorological observations at Fort Valley were 
conducted at six "parallel" stations, extending in a 
chain from the tuuber on the west side of the park 
across the [)ark into the tunber on the east side, t'on- 
tinuous records have been kept only at the three stations 
on the west side of the park. Daily readings of tem- 
perature, precipitation, relative inanidity, and wind 
mo\-ement have been made since January 1, 10fl!l, at 
these three stations, one of which is well ont in I lie park, 
one in the edge of the timber, and one about one-foui'th 
of a mile in tiie forest. Soil temperature and evapora- 
tion readings have b(>en made during the growing season. 
Additional observations of temperature were taken 



from .binuarv 



to Februarv 18 near tiie middle 



east edge of tile |)ark, and about one-fourtii of a mile 
l)ack in tlie timl)er to the oast. 

LOCAJ'IO.V AND EgriP.MENT OF METEOKOLOtiHAL STATIUX.S. 

Station 1 is located on the edge of a projecting point 
of timber on the west side of the park, at an elevation 
of 7,2(>1 fec^t. 'riH> station is ef|uipped witli a set of 
maximum and minimum tliermometers, a |)syciirometer, 
an anemometer, a wind vane, a standard rain gage, 
and an evajioration pan. The wind insti-uments and 
rain gage are connected witii ,-iii automatic register, 
wiiicli gives a continuous record of wind movement, 
wind direction, and rainfall. This station is protected 
on tiie west and northwest sides, but receives the full 
force of tiie wind from other directions, especially tiie 
sout Invest, whicii is the prevailing wind (lir(>etioii in tiiis 
region. Tlio gi'ound cover here, as at tiie park stations, 
is mainly gramma grass. 

Station 2 is situated in tiie pai'k, 840 feet southeast of 
station 1, at an elevation of 7,247 feet. Tiie e(iuipment 



consists of a set of maximum and minimum thermom- 
eters, a psychroraeter, an anemometer, a shielded rain 
gage, and an evaporation pan. 

Station 3 is in a virgin stand of western yellow-])iiie, 
1,453 feet west from the edge of tiie park, at an elevation 
of 7,348 feet. The soil at this station, as at Forest 
station (i, is a reddish clay(\v loam, inixcil with gravel and 
large volcanic bowlders. The ground cover consists of 
a sparse gi-owth of grasses and other herbs. Practically 
no leaf litter is found beyond 30 feet from the trees. 
The instnnnents are set in an opening 1(10 feet in diameter, 
and are (iO feet from tiie nearest trees. The ecpiipment 
is tiie same as for station 2, excepting that a standard 
rain gage is used. 

Stations 4, 5, and (i are temporary stations at wiiicii 
only records of temperature have been made. 

Station 4 is very near the middle of i\w ])ark, li 
miles almost due east from station 2, at an elevation of 
7,246 feet. The equipment consists of a set of maximum 
and minimum thermometers. 

Station 5 is in tiie park, 725 feet west from tiie west 
edge of the forest, at an elevation of 7,264 feet. The 
eciuiimient consists of a tiiermogi-ajili and a set of maxi- 
mum and minimum thermometers. 

Station 6 is in a virgin stand of western 3-ellovv pine, 
1,617 feet northeast from the east edge of the park, at 
an elevation of 7,330 feet. Tiicre is a gradual slope of 
about 2° from station 6 to station 5, the distance between 
the two stations being 2,300 feet. Tlie equipment is the 
same as that of station 5. 

All of the stations were located with the view of 
securing normal atmospheric drainage. 

INSTRUMENTS AND METHODS OF OBSERVATION. 

All of the instruments arc of the staiKhird United 
States Weatiier Bureau type, installed and operated 
under tlie direction of this fjureau. Since it is es])ecially 
desired to determine the atmospheric conditions to which 
seedling growth is exposed tlie instruments have in most 
cases been ]ilaced nearer the ground than is the usual 
practice of the Weather Bureau. It has been necessary, 
however, to place tlieni at a sufficient hei<jbt to avoid 
their being covered by snow, which in this locality may 
r(>ach a dejitii of 3 feet or more. 

The thermometers at the jiermanent stations are ex- 
]iosed in the standard United States Weather Bureau 
shelter, but at the temporary stati<ins they are exposed 
in improvised wooden boxes open on the nortii side. 
All of the slielters were originally placed with tiie floor 
4 feet above the ground, thus bringing the thermometers 
to a height of 5^ feet. On August 13, 1912, the shelters 
at stations 2 and 3 were raised so as to place ihv ther- 
moiiK^ters at 8 feet. The reasons for, and tlie elfect of, 
this change are discussed under Results of ()i)servations. 

Th(^ anemometers are ]ilaced 8 feet aliove tiie ground. 

Tlie evapcu'ation pans are 10 inches in de])tii and about 
3 feet in diameter. Tiiey w<'re set in a level ])osition on 
top of tiie ground. By frequent (illings tiie water was 
kept as nearly as p(i.ssil)l(^ at a constant level in all tiiree 
pans. Tiie depth of the water was measured weekly and 
from tills the loss by evaporation was coiii])iit(Ml. after 
malcing the pn)per allowance for rainfall. 

After tile first year rainfall records for the purpose of 
comparing forest and ])ark were taken only at stations 1 
and 3, .since a dilTerent type of rain gage was used at 
station 2. Snowfall iias been measured by taking the 
average depth on tiie ground at 10 pt'rnianent stakes 



near stations 2 and 3. On account of the short distance 
between stations 1 and 2, and the similarity of exposure, 
there is practically no difl'crence in the ])recipitation at 
these points, as sln)wn by the records of 1909. For this 
reason the rainfall records at station 1, and the snow- 
fall records at station 2 have been combined to give the 
total precipitation of the park. 

The instruments at the permanent stations were read 
daily between 4 and 5 o'clock p. ni. during the fii'st two 
years. For the sake of convenience the time of reatling 
was changed to between 8 and 9 a. m. in 1911. Since, 
with the exception of relative humidity, the readings 
indicate the totals or extremes for the period between 
readings rather than current conditions, relative humitlity 
is the only factor affected by the change. The tempo- 
rary stations 4, 5, and 6, on account of their distance 
from headquarters, were visited only at intervals of from 
two to four days. The use of thermographs inatle it 
possible to secure continuous records at stations 5 and 0. 
The readings from the maximum and minimum ther- 
mometers served as a check upon the accuracy of the 
thermographs, and made it possible by inter])olation to 
reduce the errors in the thermograph records in most 
cases to less than 1 degree. The records at station 4, 
since it was not provided with a thermograph and coidd 
not be visited daily, are badly broken, but furnish a 
rough comparison of conditions in the middle of the 
park with those in other situations. 

RESULTS OF OBSERVATIONS. 

AIR TEMPER.-VTURE. 

Table 1 gives a monthly summary of the temperature 
records at stations 1, 2, and 3 during the four years, 
1909-1912. A graphic comparison of the mean maxi- 
mum, mean minimum, and mean (mean maximum + 
mean minimum -f- 2) at stations 2 and 3 is given in 
figure 5. 

Table 1. — Temperature in forest, edge of forest, nnd pari;, by months, 

1909-1912. 

TEMPERATURE. 





Forest (Station 3).' 


Edge Forest (Station 1). 


Park (Station 2).' 


1909-1912 


Mean 
maxi- 
mum. 


Mean 

mini- Mean. 2 

mum. 


Mean 
maxi- 
mum. 


Mean 
mini- 
mum. 


Mean. 


Mean 
maxi- 
mum. 


Mean 
mini- 
mum. 


Mean. 


January 

February . . 

March 

April 

May 

.Tune 

July 

August 

September . 

October 

November . 
December. . 

Annual 
mean 


° F. 
41.9 
41.9 
47.3 
53.7 
65.6 
75.6 
76.5 
75.2 
70.6 
60.1 
52.2 
39.6 

58.3 


■■ F. 
16.6 
15.4 
22.6 
25.7 
31.4 
39.9 
47.8 
47.5 
40.2 
29.2 
21.7 
11.1 

29.1 


' F. 
29.2 
28.6 
34.9 
39.7 
48.5 
57.7 
62.1 
61.4 
55.4 
44.7 
36.9 
25.3 

43.7 


° F. 
42.1 
42.6 
46.9 
55.4 
66.9 
76.5 
70.6 
75.7 
70.6 
60.7 
52.9 
40.3 

58.9 


° F. 
10.6 

9.7 
19.0 
22.6 
27.3 
35.1 
45.5 
44.9 
35.6 
2.5.1 
17.2 

5.5 

24.8 


° F. 
26.3 
26.1 
32.9 
39.0 
47.1 
55. S 
61.1 
60.3 
53.1 
42.9 
35.0 
22.9 

41.8 


° F. 
42.9 
42.8 
47.3 
55.9 
66.7 
76.1 
76.7 
75.5 
71.3 
61.6 
53.2 
40.8 

69.2 


° F. 
10.3 

8.1 
17.0 
21.0 
25.1 
31.9 
43.0 
42.6 
33.4 
22.9 
15.0 

2.6 

22.7 


° F. 
26.6 
25.4 
32.2 
38.4 
45.9 
54.0 
59. S 
59.1 
52.3 
42.3 
34.1 
21.7 

41.0 



1 The thermometers at stations 2 and 3 were raised from 5J to 8 feet on Aug. 13, 1912. 

2 Mean maximum + mean minimum divided by 2. 

In order to determine whether the tem]ierature rela- 
tion between stations 1, 2, and 3 was normal for the park 
and forest of the vicinity, temperature readings were 
taken at three additional stations (4, 5, and 6) from Janu- 
ary 26 to February 18. 

Figures 6 and 7 compare graphically the current tem- 
perature at forest station 3 and ])ark station 2 during tlie 



weeks beginning December 24, 1909, and January 14, 
1910, respectively. These curves are exact tracings 
from thermograpli records. 

A comjiarison of the monthly averages as given in 
Table 1 and graphically represented in figure 5 shows a 
slightly lower mean maximum, a decided higher mean, 
and a very much higher mean minimum in the forest 
than in the park. 

The lower maximum temperature in the forest maj^ be 
attributed to lower insolation, less circulation of the air, 
and the coohng effect resulting fi'om transjnration from 
the tree foliage. The difference in maximum tempera- 
tures a])])ears to be greatest during periods of clear, dry, 
weather, notably in A])ril and May and in October and 
November, and least during periods of cloudy weather, 
particularly during the rainy season of July and August. 
Probably the greatest factor in determining this differ- 
ence is tlie presence of clouds, since under a clouded sky 
the shade of the trees would have a minimum effect. 
Measurements of the total solar radiation received near 
the ground would undoubtedly show a mucli greater 
difference tlian do the air temjieratures. Tliis is strik- 
ingly illustrated by the soil temperature (Table 4) which, 
despite the higher mean air temperature in the park, is 5° 
lower in the forest, at a depth of 2 feet, than in the park. 
At a depth of 1 inch the temperature of the soil exposed 
to the sun was found to be 140°, when in the shade of a 
group of trees near by it was only 87°. 

Another factor wliich may influence the relation be- 
tween forest and park maxima is wind. The periods of 
dry, clear weather are also periods of liigh winds, wliich 
even in the forest promote a thorough diffu.sion of 
atmospheric heat. But in the rainj^ seasons, jiarticu- 
larly in July and August when the wind is very low, the 
air in the forest, where the wind movement is much less 
than in the park, frecpiently becomes stagnant, wdth the 
result that the current temperature at the forest station 
may be higher than at the ])ark station. 

Since the extremes of forest and park temperatures 
tend to neutrahze one another — that is the maximum 
in the park is liigher and the minimum lower than in the 
forest — mean tem]ierature is a poor indicator of the ac- 
tual conditions. It is in the tempering of extremes, 
particularly the minimum, that the cUfference between 
forest and park makes itself felt. As pointed out under 
"Chniate," the fluctuation between day and night tem- 
perature is enormous. This is true of the ])ark much 
more than of the forest. It is common in clear weather 
to find tiie minimum tem])erature in the forest 10° or 
12° higher than in the ])ark, and occasionally tlie differ- 
ence is more than 20°. 

The relation between current as well as extreme tem- 
jieratures in forest and jiark is strikingly illustrated by 
the thermograph records in figures 6 and 7. The curves 
for the forest show, in addition to a much smaller dailj' 
range, less ]ironounced minor fluctuations and a more 
gradual change from one extreme to the other. As indi- 
cated on the sheets, the curves in figure 6 cover a period 
of practically clear days, while those in figure 7 cover a 
period of both clear and cloudy days. Figure 7 shows 
that the curves for forest and ]iark are practically iden- 
tical in cloudy weather, a similarity wldch is char- 
acteristic throughout the year. The differences between 
the records of the forest and the park, in clear weather, 
are less pronounced during the summer time than during 
the winter time. 

Since it was suspected that the great difference be- 
tween the minimum temperature of tiie forest and ])ark 
might be due to ground influence, owing to the fact that 




I 




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hAeqH 



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:^r" 



.:>' 




^^^- ^^'^^^ 



\ Fores 



^ 



J lu )i U ib 6 10 M -iu -rj b 10 >J 'u '^ 3 10 1^ ^0 ';3 a 10 lb 20 ii, i 10 1& £0 I'l b 10 15 2u 26 6 10 <^ 20 25 a 10 15 2o ib 5 10 1? 20 25 5 tO 15 20 26 

Fig. 5. 




Fig. C. 







/;- 1, 



t : d> t J 



*)'-' 



iXi^ 



^^ 



B^^^ ^JA^a. 



-te 



.'> .A^E 



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^ > --t > 1 V ^ V --^t^t ^. -t=^---. I ■>-^-^-^--^=^---^-l \=^=<- ^^^=:f--V 



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Fig. 7. 



0} 
1 

O 
"D 

o 
u 
q; 
q: 

d 

+- 

to 

+- 

c 

E 
\. 

Q) 

Q- 

X 

I 

-t- 



IS^t- 



a 

a. 

tn 

Q) 



IfiC 
L4D 
J2D_ 
mo "' 









2 



c^c. 



CD 
u 






IvVind . . Miivamfint 






Relative! Humidity 



40. 

.3a 



p: 
k 
is. 

i(L. 



Legend 

Forest 

ParK 



Ev(t3poratior 



Precrplitatton 



10 l^j iJ ii 5 10 1- 



li :u v'^ i lu li ;u is b 10 '5 20 Ji 5 10 li iu 2'i i IJ l^ ij 25 5 13 15 ij ii S 10 Ifr ^J '25 & K> 15 20 25 5 10 15 213 26 



Fig. S. 



25318—14 — -2 



10 



tlic shcltcre were i)liK'e(l only 4 foot above tlie pround, a 
second sot of lUiixiimiin iirul minimum llicrmomcters 
was exposed at station 2 in a shell or ])laeed at Gi I'eel 
above tlie ground during the niontlis of June and July, 
1912. A comparison of tli(> monthly average al the twn 
elevations ' follows: 

T.MU.r. '2. Tcmpfruhui nl iliffiitnt In'niJils nhnu llu (jrinuid. 



51 feet. 



June.. 
July.. 



— 


°F. 


74.'.! 1 


73. C 



i.\imuni. 


Mean in 


nimum. 
8 feet. 


S feet. 


5i feet. 


74.2 
73.1 


32.7 
40.1 


°F. 
34. n 
42.0 



Mean. 



55 feet. S feet. 



°F. 
53.8 

5G.S ; 



iVs was to be ex])ect(Hi, the results in botli months 
show a reduction of the range, due mainly to llu; higher 
minimum, at 8 feet; but the difTercnco is not sullicieni 
to warrant the opinion that the ])ast records in the ])aik 
are abnormal. On August 13, the shelter at station ;! 
also %vas raised to 6i feet. A comjiarison of tlie records 
at stations 2 and 3 before and aftcn- the shelters were 
raised discloses no material change in tlie relations. 

Since tlie relative rather than absolute teni|)eratures 
in forest and park are considered the important thing in 
this study, it has not been considered necessary to reduce 
the readings, before and after raising the thermometers, 
to the same basis. This should be considered in com- 
paring stations 2 and 3 with station 1, but since the 
temperature at station 1 is not regarded as typical of 
either forest or park, it has not been considered in this 
discussion. 

The temperature at station 1 in tlie edge of the forest 
usually lies between that of the forest and that of the 
park station, but ap|)roaclies the latter very closely. 

The records of the temporary stations 4, 5, and (5 indi- 
cate practically the same relation between forest and 
park temjicratures as that found at the permanent sta- 
tions 2 and 3, excepting for the maximum tem])erature, 
which at the temiiorary stations is higher in the forest 
than in the ])ark, while at the permanent stations it is lower 
in the forest than in the park. A ct>m])arisoii of the 
records at the six stations show that \\\\\\ res])ect to the 
mean and minimum tem])eratures, at least, the conchtions 
indicated by the pei'manent stations may be considered as 
fairh" normal for this locality. 

A comparison of the aboA'c^ results willi hiuropean 
observations is interesting. 

At stations in Europe similarly located in regard In I he 
forest, as at the Fort ^'allev Exjieriment Slalion, the 
maximum is lower and tlie minimum higher in the forest 
liian in the open; thus the general ed'ect, that <if mitigat- 
ing the extremes, is the same. The difference is that al 
the European stations the maximum is lowered more 
than the minimum is raised, while at this station the 
minimum is raised more than, tlie maximum is lowerd, 
with the result that at the European stations the mean 
temperature of the forest is lower than that in tlie open, 
while at this station the reverse is true. The greater 
decrease of the maximum temper;iture in the I'^uropean 
forests is probably explained by the fact that their forests 
are much denser tiian ours, t litis all'ording more shade 
and a gi'eater cooling effect, due to the transpiration from 
the foliage. The gix-atcr density of the European forests 

' The floors of the shelters arc 4 and GJ feet, respectively, above the ground, bnt the 
thermometers arc about IS inches higher, making the actual height 5J and 8 feet, respec- 
tively. 



would also be <>xpect(>d to produce a gi'eater elFect upon 
the miiuuium there than al Ihis station, but this is not 
the case, ft is probable that I he tremcMidous did'ercnice 
in the minimum leniperalures of Ihe forest and park at 
[•"or! \'alley is due |)aiil\' to olliei' caus(>s, which will be 
discussed later. 

In seeking to account for the great dill'erence between 
the minimum t<'mperatures of the forest and the park the 
lirst factor to suggest itself is tlu; difference in radiation. 
The change of lemperalure from day to night, owing to 
the excessive rale of radial ion in the rare atmosphei'e of 
this altilude, combined wilh the low atmospheric mois- 
ture, is usuidly very great. Il is therefore possible that 
I he ti'ee crowns iiiighl exert a retarding iniluence upon 
Ihe loss of Ileal sullicieni lo noticeably all'eet the leni- 
perature of ihe forest. Thi> o|)inion is supported by the 
fact thai ill eloiuly wcniher when radiation is reduced lo 
a minimum, ihe lowest temperatures recorded in llie 
I iniber are usually about the same as those recorded in the 
park, and on the edge of the forest, as shown by the 
iheriuograph eur\es in figure 7. It has been delermined 
by inslrumental observations that on cUuir nights tiu; 
temperature under a cover of branches near the ground is 
several degrees higher tlian that of a near-by open place. 
Observations have also shown that seedlings under a 
co\-er of branches are less susceptible to injury from 
frost lluiii seedlings on an adjacent open plot. But, 
whUe there is little doubt that the tree crowns do exert a 
retarding effect iqion radiation, yet wIkmi wc consider 
that in the western yellow-pine forest the trees actually 
cover not much more than one-half of the total area, and 
that in this investigation the for(>st stations arc placed 
in small op(>nings instead of directly underneath the trees, 
it seems doubtful whether the effect of the crowns would 
be sullicient to account for the great differences which 
have been recorded. 

The enormous ditference between the niininium tem- 
))eratures of the forest and park stations suggest the 
existence of influences other than that of the forest cover. 
Increasetl atmospheric moisture in the forest would have 
a tendency to check radiation, and thereby keep the 
minimum higher, but unfortunately the available data on 
alniospheric moisture are not suflicient to prove whether 
or not it is greater in the forest than in the open. 

Tli<> fact that the park is slightly below the le\cl of the 
surrounding thiihered areas suggests the idea that the 
cold air of the vicinity settles into Ihe park, thereby re- 
ducing the teinperalin-e below normal. To throw light 
on this (nieslion, a comparison of lemperatur<'s was mude 
lielwcen a virgin f<irest anil an adjaeeul cul-over area of 
<'cjual eleval ion during a period of almost I hree iiioul lis, 
from October I to I fecember _'."), Mill). Tlie place selected 
fortius study is aboiit'4 miles soul hwest of Flagstaff, al an 
altilude of approximately 7,f()0 fe(U. 'Fhe forest slalion 
is in a solid section of virgin' western yellow-pine, al)out 
one-fourth mile from the east line, which is the near(>sl 
exposure to nonforesled conditions. The station on tlie 
cut-over area is about one-fouiih mile east of the edge 
of the forest, surrounded by a scattering of trees left by 
Ihe hmiberman. Both the forest and the cut-over land 
have a gently rolling topogi'a|diy, and lie on llu' same 
level as the surrounding region. On both areas the 
instruments were set on slight rises, lo insure normal aii' 
drainage. 

The records al bolli stations wxtc secured by nieiins of 
Ricliards's ihermogi-aphs exposed in Inited States 
Weather fUireau shelters placed t feet ahoxc the ground. 
The in.struments were checked for current temperature 



11 



weekly, and before beginning tlie observations tliey were 
adjusted so that the average error on extreme tempera- 
tures was k^ss than 1°. Thermometers were not available 
for checldng the maximum and minimum temperatures 
during the period of observation; but in order to neutral- 
ize the efi'ect of such errors, the thermographs at the two 
stations were exchanged in the middle of tlie ])criod. 
Thus, while the actual extreme temperatures are prol)ably 
slightly in error, the com])arison between the stations is 
believed to be reasonal)ly accurate. The residts of the 
observations are given in Taljle 3. 

Tahli-; 11. — Tciiipcnilurc in a lirrjhi J'orrsl ami on (luniiliil lands. 



1 

! Moan maximum, i Mean minimum. 


Mean. 


Forest. 


Cut-over. Forest. 


Cut-over. 


Forest. 


Cul-over. 


-F. 

October 63. 1 

November' .55.4 

December 51.2 


'F. °F. 
64.0 1 31.4 
55.9 : 25.3 
51.0 ' 2:i.3 


°F. 
28. 3 
23.3 
21.2 


°F. 
47.2 
40.4 
37.2 


°F. 
46.1 
39.6 
36.1 



1 Thermographs exchanged Nov, 7. 

'J'he results show the same general relation between 
forest and cut-over area as between forest and park at 
Fort Valley; but the difference between the minimum 
temperatures at the stations south of Flagstaff is much 
less than at Fort Valley. The excess of the mean mini- 
mum temperature in the forest over that on the cut-over 
area, amounting to an average of 2.1'^ for the three 
months, undoulitedly ex])resses the influence of the forest 
in checking radiation. The smallness of this effect, com- 
pared with the results at Fort Valley, indicates that there 
is a drainage of cold air into the park at Fort Valley. 
But, since the inunediately surrounding forest is decidedly 
below the counti'y back of it and only a few feet above the 
pai'k, it seems that any cold-air drainage from the sur- 
rounding moimtains and mesas should make itself felt 
at the forest as well as at the park stations. All of the 
stations, both in the forest and in tlie ))ark, are situated on 
slight rises above the immediately surrounding country, 
anil therefore they should not be subject to local gather- 
ings of cold air. Tliis points Id the existence of still 
other factors. 

One explanation of these conditions lies in the |)ossii)le 
mechanical resistance offered by the forest to the atmos- 
pheric drainage from the surrounding mountains and 
mesas. The cold air of the mountain slojies would 
naturally settle into the valley below, thereby causing a 
general movement toward the ]iai-k. This air in ])assing 
over a timberetl area might lie prevented to a certain 
extent by contact with tlie tree crowns from mixing 
freely with the warmer air below, and thus be ilivcrted 
over the tree tops into the open park. In order to deter- 
mine whether the tree crowns exert any a])j)reciable 
influence in retarding the diffusion of the cold air into the 
lower strata of the forest cover, it woidd be necessary o 
secure observations on temperature at different heights in 
and aliove the forest. 

Whatever may prove the correct explanation of the 
relation between forest and park temperatures in this 
locality, it "n'ill probably be found that a considerable 
influence is exerted by the surrounding mountains and 
high mesas in the drainage of cold air into the park. The 
importance of the effect of this factor in the forest, how- 
ever, is probably mitigated by the foi'est canopy in 
diverting the cold-air currents. Moreover, the existence 
of these factors does not render the conditions in the 



Fort Valley park very abnormal for this region because 
numerous mountains, though of less height than the San 
Francisco j\Iomitauis, are scattered over the whole forest, 
and the parks are almost invariably lower than the sur- 
rounding tim])ered areas.- It is therefore probable that 
the investigation of other parks in this region would give 
results very similar to those here recorded. 

Prol)al)ly the best way to determine alisolutely 
whether the presence of the trees is responsible for the 
mdder temperature on the forest area would be to clear- 
cut a portion of the forest and compare the temperature 
relations between a station on this area and the present 
forest and park stations before and after cutting. The 
expense of such a clearing woidd not be justified at the 
present time. It is therefore the ]ilnn to wait until the 
area is cut by the lumberman. The southwest quarter 
of the section on which forest station No. 3 is located is 
owned by private interests and will probably be loggcfl 
withui 10 years. A station has recently been established 
well within the boundaries of this quarter section and about 
one-half mile from forest station A^o. 3. Observations 
will be continued at this and the other stations through 
the present j^ear, after which the^- wUl lie temporarily 
discontinued, except at the permanent forest station. 
"Wlien cutting is anticipated the observations at the tem- 
porary forest station and )iark station No. 2 will be 
resumed with the purj)ose of securing records for a year 
immediately before and after cutting. 

SOIL TEMPER.\TURE. 

Soil temperatures have been measured in the nark for 
a short period only. Table 4 gives a comparison between 
the forest and the park temperatures at a depth of 2 feet 
from July 1 to October 30, 1911. 

Tablk a. — iS'o// tfinpf'ntliiif in Joirst iin<l piirk. 





Mean temperature. 




Park. 


Forest. 


.lulv 


°F. 
61). 8 


°F. 

54.9 




1)2. f> 


57.4 




60.4 


.55.7 


October 


53.0 


49.0 








Mean for 4 months 


59.35 


54.25 



The readings were taken between S and !) o'clock a. m. 
The thermometer in the forest was located on the north 
side of a group of trees, and therefore represents the 
lowest rather than the average temperature in the 
forest. The results show very strikingly the effect of the 
trees in cuffing off the direct sun's rays from the ground 
beneath them. If the forest canopy were continuous, 
we should expect the same effect all over the forest floor. 
Undoubtedly the cooling effect may be ascribed ]inrtly to 
the leaf litter, which acts as an insulator. No data com- 
paring forest and park soil temperatures during the winter 
inonfhs are available; but a comparison of the depth of 
frozen soil is significant. On the afternoon of February 
IS, 1913, holes were dug in the middle of an opening and 
on the north and south sides of a grou]) of trees. The sun 
was warm, and all the snow had melted on the south side 
and underneath the trees, but on the north side and in the 
opening the snow lay from 6 to S inches deep. In the 
opening the soil was frozen from the surface to a depth 

2 This does not apply to the large open areas known as prairies. 



12 



of 23 inchas. On the north side of the trees it was frozen 
from the surface to a doptli of KU- inches. On the south 
side every 1i-aco of frost had disappeared. The com- 
paratively sliailow doptli of tlie frost on the nortii side is 
probably due to (lie protection of deep litter, which at 
this point was about 2 uiclies. The presence of tree 
roots may also bo a factor wortli considerino;. On the 
same afternoon the soil in the park was found to be frozen 
from tlio surface to a deptli ot L'9 inches. 

Accordini^ to Nisbot,' European observations jiavc 
shown that the nican annual tcmpcralurc of the soil is 
at all dcptiis of observation, e.\t('ndin<;- down to 4 feet, 
cook-r in the forest than in tlie open, and tiuit this dif- 
ference is greatest in sunnner and least in winter. The 
mean annual difference at a depth of 2 feet is 1.9° F. in 
Prussia, 3. .5° in Bavaria, and 3.2^ in Wurttember<j-. 

I'HECn'ITATION. 

The mean monthly and total |)recipilation in forest 
and park durinij- the four years litOd I'M 2 is <i-iven in 
Table .5. The yearly disi ribul ion is i;raphicnllv repre- 
sent (hI in liguri^ S. 

Table 5. — Precipilalion in forest mid park. 



Precipitation. 



Moan, l'JW-1'Jl-.' 



I'aik. Forest. 



Mean, 1009-1912. 



Inches. Inches. 

January i 2.30 2.14 

Kcbriiarj- I i.eg i.oi 

March 3.27 3.49 

April 0.74 1 U.G7 

May 0.25 I 0.21 

June 0. 41 ' 0.54 

July 3.11 3.45 

August 3.00 I 3.1.S 



September 

October 

November 

December 

Total 

Nov. 1-Apr. 30. 
May 1-Oct. 31.. 



Precipitation. 



Park. Forest. 



Inches. 
1.26 
1.94 
0.71 
l.Sl 



Inches. 
1.43 
1.92 
0.69 
1.4S 



20. 19 
10. 22 
9. «7 


20. SI 
10. OS 
10.73 










A (■oinparison of precipitation records in forest and 
park is of value only in so far as if indicates the influence 
of the forest canopy upon the normal descent of rain and 
snow to llie ground. The records show, as a rule, a slight 
excess of i-ain in the forest during the summer time, iuid 
a smaller excess of snow in the park during (lie winter 
innc. The apparent excess in the forest" during the 
sunmier months is generallv due to the fact that th(> iniii 
gage in the forest, being" sliellered from the wind, iv- 
eeives more nearly the total precipitation than docs the 
.irage expose.! to the wind in the park. .MonM,\(.i-. ih,. 
rain gage in the forest, being slaticuied in .-m opiMiiiii;-, 
undoubtedly registers a heavier rainfall than would be 
the case if proj>cr representations were given to the areas 
under the tree crowns. In other words, the present nun- 
fall records for the forest are too higli. The slight excess 
of snow shown in the park during the winter months is 
actual. Snowfall is measured by taking tlie av(>rage 
de])th on the ground under the t'recs as "well as in the 
openings, and since more or less snow is interce|)ted bv 
the tree crowns, the total amount reaching the ground 
in the forest is usually somewhat h^ss tluin in the park. 
Although the actual precipitation, especially in the lime 
of thunderstorms, may vary considerably "between tlie 
forest and park stations for individual" storms, these 

The Forester, vol. 1 .p. 71. 



differences are usually equalized during a season; and 
while the yearly records show a small ilifferonce, it is 
doubtful \vhether there is any actual difference in the 
amount of rain wliich falls from the sky over the forest 
and the park during a long period of years. 

Some idea of th(> influence of tlit^ tree crowns in inter- 
cepting rain was gained by placing one rain gage under 
ii group of trees and another in a near-by opening durini,^ 
the month of August, li)()(), when the rainfall was excep" 
tiomilly hciivy. The results show a total of .").7-l inches 
in the ojx'iiing against 3.44 inches under the trees. 
Mence the tree crowns intercepted 40 per cent of I lie rain 
which fell (m them. I'ndoubtedly a considerable portion 
of the water intercepted bv the trees drains off, falling 
on the ground underneath the outer portion of the crowns 
and causing an excess here, which in part makes up for 
I lie delicit directly under the trees. But it stands to 
reason that for a given rainfall the total amount of water 
retiching the ground on a given area in a forest is less 
than what would reach the ground on the same area if 
the trees or other vegetation were absent. 

Observations on snowfall show that a large timouni of 
snow is collected by the tree crowns, especially in still 
weather and when the snow is moist. Usually the bulk 
of this snow is blowm off and deposited in the near-by 
openings, while the remainder melts and reaches the 
.ground in the form of water, or is lost by evaporation. 
As a result of these conditions the snow is usually much 
deeper in the small openings than directly under the 
tr(>e groups. After several days of melting it is common 
to find the ground under the trees entirely bare fsee 
fig. 9), while in the neighboring small openings it is from 
2 to 3 feet deep. Usually the average depth of snow in 
the forest is somewhat less than in the ])ark durimr the 
winter because of the more rapid melting result ingl'rom 
the higher temperature in the forest: "but .systematic 
observations during four years .show that in tlie spring 
btiiiks of snow remain in the forest from two to three 
w(>eks after ;dl traces have disappeared from the pari-'. 

There is also a noticeable difference in the amornt of 
I'un-olf. While no actual measurements of run o(f have 
been made, general observations show bevond anv doubt 
that the |)er nuit of run-o(f is greater in the nark'than in 
tli(> forest. The park soil, as well ;is tluit in tlie openino;.., 
ol the forest, remains frozen to a de))th of 2 feet or more 
through the winter, with the result that it \< m a poor con- 
dition for .absorbing moisture: but under the trees, partic- 
:daily on t he sout li side, the ground i^ often fo-ind entirelv 
frei' (,f frdst in midwintei-. ' .\.s the snow melts on the.se 
spots, it is com|)letely absorbed by the soil and leaf litter. 
.\o| only this, bill Ihi' water froni melting snow banks in 
ailjomuig openings where the ground is frozen runs into 
these spots and i-; largely absorbt-d, thus forming V(U-itab!e 
reservoirs. This process was oliserviMJ to be sroing on on 
the ISlh of Febrnai-y. 1913, when, though t he frosl^was 23 
inches deep in the openings of the forest tiiid 29 inches 
dt-ep in the park, not a trace was found on the s,).|th side 
of t he huge t ree gronjis. 

AT.MOSI'HKIUC HUMIDITY. 

Table (i gives the nie;in inoiithlv relative hnmiditv at 
each station from 1909 to 1912.' inclusive. The same 
data are represented graphically in figtire S. 



13 



Table 6. — Relative huinidity. 



Month (average 1909-1912). 



January 

February 

March.. ^ 

April 

May 

June 

July 

August 

September 

October 

November 

December 

Mean Jan. 1-Dec. 31. 
Mean Nov. 1-Apr. 30 
Mean May 1-Oct. 31., 



The annual means at the three stations do not show a 
siiHicient dilFerence to warrant any conchision with re- 
spect to difference between forest and park. 



Mean relative humidity. 




(Percent.) 




Forest (3) 


Edge of 
forest (1). 


Park (2). 


Per cent. 


Per cent. 


Per cent. 


OS. 8 


71.3 


69.3 


57. S 


65.3 


64.9 


00.3 


6.5.4 


66.5 


47.9 


45.5 


45.0 


3S.0 


30.4 


33.9 


34.9 


29.1 


33.9 


69.9 


52.8 


50.3 


00.3 


57.8 


61.0 


.52. 7 


48.8 


50.3 


.52. 


51.2 


53.8 


57.3 


58.9 


61.1 


48.0 


44.8 


54.1 


53.7 


51.8 


54.3 


50.7 


58.5 


011.2 


50.7 


45.0 


48.3 



The records indicate a lower relative liuiuidity in the 
forest than in the park during the winter, and the reverse 
relation during the summer. Since humidity readings 
have been taken only once per day and not simultane- 
ouslj' at the three stations the data at hand are con- 
sidered inadequate to furnish a reliable comparison 
between the forest and the park. Such comjiarisons of 
absolute humidity as have been made show a small diU'er- 
ence in favor of the forest; but, as in the case of relative 
humidity, the difference is too small to warrant any con- 
clusions. An adetjuate study of atmosjiheric moisture 
would require several readings during l)otli day and night, or 
else the use of self-recording instruments. With respect 
to absolute humidity the results of all observations made 
fail to show any marked or constant difference between 
tile forest and the open. 

WIND. 

Table 7 gives the mean daily wind movement by 
months in tli(> foi'est, the edge of the forest, and the park 
during the four years from 1909 to 1912 inclusive. A 
graphic comjiarison of the forest and the ]iark is given in 
fisrure 8. 




Fig. 9.— Distribution of snow in the forest. Note bare ground under trees (looking north). 



Table 7. — Wind movement in forest, at edge of forest, irnd in park. 



Average 1909- 
1912. 



Mean daily wind move- 
ment. 



Forest 
(3). 



Edge of 

forest 

(1) 



Miles. 

Januarv 56. 5 

February 60. 9 

March 68. 2 

-Vpril 80. 6 

May 84. 9 

June 76.5 



Miles. 
1 93. 2 
111.1 
121. 1 
150.6 
167.2 
150.0 



Park 
(2). 



Miles. 
107.3 
126.5 
130.8 
158.7 
174.2 
151.9 



Average X909- 
1912. 



Mean daily wind move- 
ment. 



Forest 
(3). 



1 



, Miles. 

July 50.5 

August 49.0 

September 58.9 

October 59.2 

November 59.8 

December : 56.2 



Edge of 
forest 



Miles. 
98.7 
84.9 
103.3 
117.2 
1 113.5 
95.2 



Park 
(2). 



Miles. 
106. 8 
86.8 
107.0 
US.O 
lll.K 
105. 8 



The wind movement is about twice as great in the 
park as in the forest. This difference is in accord with 
results obtained at European stations. The protective 
influence of the timber is undoubtedl}' the only inipoi-tant 
factor to be considered in accounting for this ilifl'erence 
between forest and park conditions. 

The wind movement at station 1, in the edge of the 
forest, is but slightly less than tluit at station 2 in the 
park. The smallness of this difference is due to the fact 
that station 1, while protected on the west and northwest 
sides, receives practically the full force of the wind from 
other directions, especially the southwest, whence come 
the prevailing winds of tliis region. 



1 Only 3 years records. 



14 



KVAI'DliA'l'IDN. 



Table 8 gives the monthly evjiponitiou in tlus forcist 
and the edge of the forest from 1!)U9 to 1912 inclusive. 
A graphic comparison of the two stations is given in lig- 
ure N. Since the evaporation pans would fn^eze solid in 
the winter time, measurcinctnt^ have Ijihmi madi; only dur- 
ing tlie growing season, from May 1 to Octnlxu' Ml. In 
llioi), the measurements were not begun until July I, and 
tiierefon^ the monthly nic.-ins for May and June represent 
onlv three vears. 



Tahi.k S. — Eritporalioii in niiil iil nhji i>f forest. 



Total evaporation. 



Total evaporation. 



.^vcriige, iyO!)-I9r2. i I .\verat;e, I'.td'.i I'.il.'. 

Forest Edge or , i Foreist 

(3). forest (1). , (3). 



.May 


5.S 
T.3 
5.5 
4. S 


s. s 
10. u 
7.5 
11.1 




Inches.' 
4. 2 


October 

Total 


3.2 


July 






.■VugTist 


....! 30.8 



Edge of 
forest (1). 



!■. 1 
4.4 



43. s 



' Trom free water surfaee. 

The factor mainly res|)(iiisii)le for ihc decreasefl t!v;i])- 
oralioii in the forest at this station is probably tlu; dv- 
creased wind movement. Other factors affecting evaj)- 
oration are atmospherics moisture and temperature. Tlu^ 
rcH'ords show a slightly higher relative humidity in Lhi! 
forest than in the ]iark during the ]ieriod when evapora- 
tion is measurc^d; but, as already stated, the data on 
atmospluiiic moisture tuv inadc^piate for an accurate com- 
parison of forest and park conditions. The higher tem- 
|)(!rature in the forest would tend to increase the evapo- 
ration, but since this occurs mainly at night when tlu* 
atmosph(!re is comparatively near the saturation ]ioint, 
its inlhuMice is wmivc than olfsct. by the lower wind move- 
ment. 

An e.\aminat imi of ligiire S shows iJiat the eva|)oratioii 
is the greatest during tli(^ months of May and June, 
which are also tlui months of greatest wind movemeiit, 
lowest rtilative humidity, and lowest precipitation. In 
July and August, although tlui t(iinperature is liightu- flutn 
in the preceding months, the evajioration is lower on 
acccjunt of hnver wind movement and the higher relative 
humidity and precipitation. Since the summer rains 
usually do not begin until tibout July I."), the records 
during the first half of tlu! month arcs luucli higher than 
during the latter half. The low evaporal-ioniiiSeptiMiiber 
and October is due mainly to a decretise in tompisratiire, 
since wind and relative humidity during these months 
iiro favorable for high evaporation. 

The evaporation in the forest during the growing season 
is only 70 per cent of that at the edge of the forest. 
lOvaporation was measured in the park (station 2) only 
during a period of four months in 1909. During this 
time the record was slightly Ixilow^ that at the edge of t he 
forest. Since the two stations an; so nearly alike with 
rtispect to t(!m[)erature and wind (!\posure, the record 
at the edge of the forest is considered re])resentati\e of 
the park. 

(iorman observations ' at 17 stations for 10 years show 
the evaporation from a frise water surface in the forest 
from May 1 to October ol to be approximately 40 ptsr 
cent of that in the open. The great reduction of evap- 
oraticui in the (ierimxn forests .is coni|)ared with our yel- 
low-pine forests is explained by the greater density of the 

' .M. \V. Harrington, Forest Servi<'e liiilletin No. 7, part 2, p. 97. 



foi'iiier. -V further explanation is found in the fact that, 
at tilts (ierman stations the moan ttimjxu'ature in the for- 
(sst is lower than that in the open, w hile lusre it is higher. 
The decrease of (svaporation in th<! forest may be rts- 
garded as expnsssing the sum total of the forest's influ- 
ence uixui wind, relative humidity, and to a less extent 
of solar radiation. The latter factor is expressed only 
in a. iiiincir degrees, because the evaporation pan in the 
forest, is statrontsd in an opening where it is shaded by 
tins trees only during a few hours of tlus day, and not at 
all when the sun is at its height. 

sr.M.M.VUY OF KKSlT|/rs OV MKTKOKOl.OlilCAI, STTDY. 

The records show that the me;in .■iiinnal temperature 
in the forest is 2.7° higher tlitui in the park. Tlus most 
imjiortant dilference, however, is in the extreme tem- 
peratures. The maximum averages 0.9° low-er, and the 
minimum ().4° higher in the forest than in the park. The 
mean daily range is 7..')° smaller in the forest. 

The evidence at hand supports the opinion that the 
above relation is due to the influence of the forest canopy. 
The influence upon minunum temperatures, which is by 
far the gi'catest and most important, appears to be exer- 
cised partly by the action of the tree crowns in checking 
the loss of heat by radiation, but mainly by the deflection 
of cold air currents from surrttunding mountains and 
high mesas. A final and absolute determination of the 
extent to which the presence of trees is responsible for the 
amelioration of temperature conditions in the forest, and 
the determination of the manner in which its iiilliienc(> is 
exertcnl demands further investigation. 

The temperature of the soil in the forest during the 
summer, wlien shaded by trees, is about r->° lower at a 
depth of 2 feet than at the same depth in the park. 

The available dtita on atmospheric humiility show no 
definite ndations between forest ami ]iark, excepting that 
the relative humidity appears to be slightly higher in the 
forest during the summer time and slightly higher in the 
park during the winter time. 

The only appreciable effect of tlie forest upon precipi- 
tation is ill its influence upon the distribution and disposal 
of snow and rain. Considerable amounts of snow and 
rahi are coUected by the tree crowns. Probably the 
gi-eater portion of this blows or drips off, but an appreci- 
able amount is lost by evaporatit>n. The snow faUs 
more evenly and accmuulates to a slightly greater depth 
in the park than in tlie forest in the winter time, but it 
remains on tlie ground from two to three weeks later in 
the forest. A greater proportion of the snow waters is 
absorbed by the soil in the forest than in the park. 

The average wind movement is only .'il per cent as 
gi-eat in the forest as in the park. 

The evaporation from a free water surface is only 71) 
per cent as grctit in the forest as in the ));irk. 

The influence of the forest u])on all factors studied, 
with the exception of rtdative humidity for which our 
data are inadequate, is similar to that shown by Euro- 
pean observations. In the European forests, as in those 
covered by this study, the extremes of tvinperature are 
modified ; but in European forests th<> maxima are low(>red 
more than the minima are raised, with the result that the 
me.an temperature is lowtM- in the forest than in the open, 
while in our forests tins maxima are lowered less tluin the 
minima are raised, with the result that the mean tem- 
perature is higher in the forest than in the open. The 
influence of the forest hi decreasing evaporation is 'M) per 
cent gi-eater in Europe than is shown in this study. 



15 



SILVICULTURAL APPLICATION OF THE METEOROLOGICAL 

DATA. 

From the first introduction of scientific forestry in tliis 
region it lias been recognized that cUmatic conditions of 
inuisual severity must be dealt with. The problem of 
reproduction has been and will continue to be the gi'eatest 
silvicultiu'al pi'oblem in the management of our western 
yellow-pine stands, which constitute 95 per cent of the 
timber forest in Ai'izona and New Mexico. Recognizing 
these conditions, a shelter-wood system of cutting, in 
which a portion of the original stand is left not only for 
the production of seed, but also as a protection to young 
growth, has been used since the forests came under the 
administration of the Forest Service. This investigation 
shows that extremely adverse clunatic conditions exist, 
that a forest cover exercises a profound influence in 
ameliorating these conditions, and that the reasons for 
ailopting a shelter-wood system are well founded. 

FORESTATIOX OF PARKS. 

As regards parks, this study indicates that climatic 
conditions on such areas are decidedly unfavorable to the 
establishment of forest growth; that the invasion of such 
areas by forest trees can as a rule not be expected, and 
that if theu- afforestation is ever accomplished it must be 
by artificial means. 

It is believed that atmospheric conditions hi the Fort 
A^alley Park are fairly representative of other parks in 
the region. With respect to wind and consequent evapo- 
ration there can be no great difl'erence. In larger parks 
these factors will be higher and in smaller parks lower 
than at Fort Valley. They will also vary with the loca- 
tion in the park, the lowest wind and evaporation being 
found along the south and west borders, since the pi-evail- 
ing winds are from the southwest. We might expect a 
great variation with respect to minimum temperature, 
but smce nearly all parks are so situated as to receive 
cokl-au' drainage from the surrounding country, very low 
minimum temperatures resulting from this cause as well 
as from radiation may be looked for. On prah'ies, which 
are on the same level as the surrounding timbered country 
and are thus exempt from cold-au- drainage, the only 
reduction of minima would be that due to radiation, and 
here we may expect to find about the same temperature 
conditions asontlie cut-over area descril)ed on pp. 1622- 
1623, namely, a shgltt increase of the maxima and a re- 
duction of the minima by 2° or 3° as compared with the 
forest. 

The action of drought, through excessive evaporation 
from the soil and from the plant itself, and of low tem- 
perature in the form of unseasonable frosts, is very de- 
structive to young seedlings before they have developed 
deep-root systems and woodv stems. Annual observa- 
tions on marked plots on tlie Coconino and Apache ' 
National Forests show that even under the partial pro- 
tection afforded on cut-over lands over 95 per cent of 
the seedling-s die during the first year. The loss among 
seedlmgs which might sprmg up in a park under full 
exposure to the wind and frosts can only be conjectured. 

A practical demonstration of the protective influence 
of the forest upon the action of wind and evaporation is 
afforded bj- a planting experiment in the spring of 1909. 
Fifty 2-year-old western yellow-pine transplants were 
planted on April 22 under similar conditions at station 1 
on edge of the forest, where wind and evaporation are 
practically the same as in the park, and at station 2 in 
the forest. Western yellow pine at this age is not verj- 

' Forest Service Bulletin 125, pp. 23-28. 



susceptible to frost, and therefore tlie losses are attrib- 
utable to other causes. On May 21 only 11 per cent of 
the plants at station 1 were alive, while at station ?>, 90 
per cent were alive. Soil samples taken on ilay 21, how- 
ever, showed a moisture content of 17 percent at station 
3, and 17.5 per cent at station 1, in the zone occupied by 
the roots, which were about S mches long. The moisture 
content is abundant for pine seedlings on both soils under 
reasonably favoral)le atmospheric conditions. No evapo- 
ration records are available for this period, l)Ut the wind 
mo^•ement at the edge of the forest was almost twice as 
great as in the forest. The conclusion is therefore that 
tli3 plants in the edge of the forest, with an abundant 
water supply at then- roots, dried up simply because the 
water loss from the foliage exceeded the amount supplied 
by the roots. In the forest, on account of a lower rate of 
evaporation from the foliage, a safe ])alance lietween the 
supply and expenditure of water was maintained. 

As a result of tlie above experience, planting in parks 
was almost despaired of, but another attempt was made 
m 1912 with excellent results. This success is attributed 
entireh" to the use of a superior grade of plants. It has 
been demonstrated that western yellow-pine can be 
planted successfully in parks if good stock is used; but 
we have no assurance that plantations will grow to matur- 
ity. It is possilile that the absence of natural forests in 
parks may be explained by the presence of adverse atmos- 
jjheric conditions which make it impossible for seedlings 
to survive the fii'st year; but it is proba])Ie that there are 
also other factors, such as unfavorable soil conditions. 
Tlie 1)ulk of our park areas in the yellow-pine t^'pe will 
eventually be given over to agriculture; but such portions 
as are too rockj^ for farming should be forested if possible. 

FOREST NURSERIES. 

The nursery of the Fort Valley Experiment Station is 
at present located at tlie edge of the forest. It is pro- 
tected from sout Invest winds, but temperature conditions 
approach those of the park. The site has proven an 
excellent one for western yellow-pine: but Douglas fir, 
Jeffrey' pme, Austrian pine, European and Siberian larch, 
and several other exotics have suffered severely from 
winter killing. A small Douglas fir nursery has been 
established in the forest, where it grows naturally. Should 
any of the other species prove desirable for planting, 
nursery stock could undoubtedly be produced more suc- 
cessfully in the forest, where it will be less exposed to 
frost. 

REPRODUCTION AFTER CUTTING. 

The pai-k and vu'gin forest represent the two extremes 
of exposure and protection. Conditions on cut-over 
lands are intermediate between the above extremes. 
^Moreover, it should always be borne in mind that con- 
ditions on any area vary with the degi'ce of cutting, 
topogi'aphv, and other local factors. It is beheved that 
the relation between park and forest at Fort Vallej- 
will be found to hold, in a general way, for the whole 
Southwest: but we should not be surprised to find con- 
siderable modifications of this relation in one direction 
or another m different locaUties. 

The pi'unary object in seeking to take advantage of 
the protection afforded by the forest is to secure natural 
I'eproduction. If planting were resorted to, it is possible 
that there would be no need for leaving trees to furnish 
protection for young gi-owth. But suice planting does 
not promise to become economically practicable under 
general conditions, natural reproduction must be the 



16 



first consideration in any >il\ icultnrai system whicli we 
may adopt. 

Areas wliieii have i)een lo<^<j;e(i-o\('r approach I he 
state of parks in proportion to the desiri-ee of eutting, 
with this dilfcrcnce, tiial cnt-over areas, unless, hive 
tiu> parks, they lie below the surrounding country, are 
less subject to" unseasonal)le frosts. E.xcepting hi val- 
leys which receive cold-air drainage, the did'ereiice in 
niiniiiiuni leni])eralures on heavy and light, cutlings, 
judging from the relation i)etweeii virgin forest and elixir- 
cutting descrii)ed on ])ages 1622-1()23, proliably docs 
not orchnarily exceed one or two degrees. But we may 
always expect a v(>rv pronounced (Hll'erencc wilii re- 
spect to wind movement, particularly on long stretches 
of comparatively level country. A comparison of the 
wind movement on a cu(-over area at Fort "\'allev, 
IVom which aiifiroximately two-tiiirds of tiie original 
stand lias heiMi removed, with the vh'gin forest and tiie 
])ark from .Iun(> 12 to July 12, 1910, showed the follow- 
ing average daily wind mo\ements: Forest, TM.o miles: 
cut-over area, S(i.(5 miles: |)ark, 140.2 miles. In ri^gions 
of rough to])ograjiiiy little concern need he felt lor tlie 
effects of wind in narrow valleys and on noi'th slopes: 
but on exposed ridges and south slopes exposure to 
both wind and sun must be rigidly gu.-irded against. 
V;illeys, while protected from wind, are more or less 
sni>ject to frost, and if tluMC is any evidence of frost 
injuiv liea\y cutting siiould i)e avoided. 

(ienerally, young growth estal)lisiied under tiie ])rot(>c- 
tion of older trees is often not considered saf(> from cli- 
matic injury even after it is '> or 6 j'ears old. Tliis a)i- 
plies particularly where the sudden removal of tiie cover 
is involved. Tiiis danger is indicated hy an experiment 
on the Coconino National l*\)rest. 

On a section near Flagstafl' where .-dKnit two-thirds of 
the stand was cut 27 years ago, a considerable aiiKiunt 
of young growth came in from 4 to 8 years ago. In 
order to observi' the progress of reproduction, two ])er- 
nianent samjile plots were eslablished on the area in 
1!)0S, the seedlings being liien from 2 to 4 years old. 
A jilot was also estalilislied in a neigliboring virgin stand. 
Counts from lOOS to I'.iil showed the following losses 
from year to year: 

T.\i)i,K !). Loss of seedlings for period of three years. 





Loss. 




Cut over. 


Poorest. 


lfl08-09 . . . . 


Per cent. 
21.3 
20.5 


Per cent. 
■'fi ^} 


I'.Kl'J-lO 


IQ 9 


lylO-l 1 


12. 1 91 n 









Til the summer of 1911 the area was logged a second 
lime, removing the entire stand. Jn tlie summer of 
1912, when the area was examined again, it was found 
that a large number of sfiedlings wc^re dead or dying. 
Since the plots were tlistribuled in the logging opiM'ations 
the percentage of loss from 1911 to 1912 could not bo 
accurately determined, but it was possihlr? to 'r^i a 
conservative figure from the a(-tual number of dead 
seedlings found. In order to secure more representative 
conditions, additional plots were examined. Counts 
showed that out of 700 seedlings from -i to S years old, 
2.3 per cent were dead and 13 [ler cent doubtful. Addi- 
tional plots were also laid out in the forest which had not 
been disturbed bv cutting. Out of (iOO seediiiii's on 



these plots only (i per cent were dead and 2 per cent 
doubtful. I'lu'. hoav}' loss on the logged area (it is safe 
to count 10 per cent of tiie doul)tfiil seedlings as dead) 
is imdoubtiHliy due to tin; effect of this sudden exjiosiire 
incident to heavy cutting. 

"^riie idea of supplementing the protection afl'orded by 
standing trees by .scattering branches cut from felled 
trees upon the ground has been suggested and has many 
adherents. Flxperiments ' have shown that a brush 
cover increases tiu^ teiii])erature by two or three degrees 
on cold, clear nights, ami that it is effective, within certain 
limits, in sa\ing siH'dlings fnnn frost damage. Olisei'va- 
lions also show that a brush cover exercises an ap|)re,- 
ciable influence in preserving soil moist un;, although 
this effect disapjiears after protracted periods of drought. 
Des|)ite theso influences, annual observations on an area 
of 12.") acres on which brush was scattered after logging 
in 1908 show a greater lo.ss of seedlings and a smaller 
number remaining at the ])r(went time under brush than 
on adjoining open spots. This result may be ])artially 
explained ])y the fact that many of the seedlings which 
come u]) under tlii^ brush receive too little sunlight, and 
the fact that after file first year the needles drop off, 
forming a litt(U' ujion the ground which makes it impos- 
sible for seeds to reach the mineral soil. But the effect 
of a brush cover is far-reaching, and a number of years 
must elaj)S(^ before it can be definitely determined what 
the net result will be. As the leaf litter decays it will 
improve the soil, while the coarser branches will form a 
light sci'een, though not casting an excessive amount of 
sliade. On the other hand, the dry needles and branches 
are always a lire menace which may cause the destruction 
of an entire crop of seedlings. 

A ])roposed modification of the l)rush cover, which 
aims to secure a large share of the advantages without 
the attendant disadvantages, is to scatter large branches 
rather thinly upon tlie ground, so that clear spaces a foot 
or two in diameter will intervene. Conditions directly 
underneath the branches will be the same as describetl 
above: but seeds falling in the open spots will come in 
contact with the, mineral soil, and the seedlings while 
enjoying sufficient sunlight will receive a considerable 
amount of wind protection. The fire danger is decreased 
but not eliminated. 

Another method is to leave the tops practically undis- 
turbed after removing the merchantable logs from the 
felled trees. Due precaution being taken not to leave 
tops where they will constitute a fire menace to e.xisting 
advance growth, the prostrate trunk with its projecting 
braiK'hes thus forms a sun shield and wind break for 
seedlings which spring up underneath and on the shel- 
tei'ed side. This practice is the one followed on all tlu^ 
old cuttings under private control, with the exception 
that in such cuttings no attention was paid to fire danger. 
Many instances have been found wlier(i good reproduction 
has succeeded these cuttings, and not infrerpiently it is 
confined almost entirely to rows of saplings along old 
tree trunks. There are, however, plenty of instances 
wher(^ j)raclically no young growth has started on old 
cuttings, not even along the tops. Among tiie factors 
which may lie responsible for this condition are fires, the 
destruction of seed by rodents, or, in the case of iieavy 
cuttings, absolute lack of seed. The main advantage 
of this m(^lhod over scattering the branches is that of 
economy, since the only cost is that of jmlling an occa- 
sional top away from a standing tree or grouji of saplings, 
while tlie cost of scattering the branches amounts to 



• Forest Service Circnilar 174, pp. S-10. 



17 



about 40 cents per thousand board feet of timber cut. 
The protective influence under tliis method is concen- 
trated on limited strips along each tree top, while, where 
the branches are cut off and scattered, their influence is 
spread over a larger area. 

The shelter necessary to insure conditions favorable 
for natural regeneration is not usually secured without a 
sacrifice of nierchantalile timl)er. The least dillicult 
stands to regenerate are those having a large j^ercentage 
of immature trees at the time of cutting, lender th(> 
present system of cutting, it is the aim to retru'n in 
approximately 40 years and remove the then matiu-e 
trees which at the time of the first cutting were left for 
the piu'pose of re])roduction. It is also the aim to leave 
enough trees so that the yield at the time of the second 
cut will warrant the expense of logging. Usually aljout 
one-third of the stand, or from 2,000 to 3,000 Ijoard feet 
per acre, is left. If this timber consists of immature 
growing trees they can bo left at a profit. Occasionally 
there are enough such trees in a stand to jirovide for 
reproduction, and then the cost of regeneration is nil. 
But often we have the other extreme in which the stand 
is made up almost entirely of mature and overmature 
trees which would yield the greatest return if cut now. 
If we leave a sufficient number of such trees to furnish 
the seed and protecticni recjuired for reproduction, a 
certain amount of deterioration will take ]ilace before the 
second cutting can be executed, and the financial loss 
through this deterioration must 1)0 charged against the 
cost of regeneration. Usually we find the intermediate 
condition where there are a sullicient number of imma- 
ture, or at least sound and thrifty trees, so that it is 
necessary to leave only a few mature trees of high merchant- 
able value. However, whenever it is necessary to leave 
a tree which at the time of the second cut will not yield a 
net return equal to its present value, considering growth 
or deterioration, increase in stumpage price, and interest 
on the capital, that tree is left at a financial loss. 

In cutting stands in which, to satisfy the requirements 
for natural reproduction, it l>ecomes necessary to leave a 
large volume of matiu'c timber, the old question arises, 
"Will the service rendered by these trees through the 



production of seed and protection to young growth offset 
the loss which wall l^e incurred through their deteriora- 
tion?" and "Would it not be more economical to cut 
these trees and regenerate the area by planting?" 
Considering the care required in planting western yellow- 
pine in this region, the cost per acre can probably not be 
reduced below $\2. If in cutting it is found necessary 
to leave four mature trees per acre, aggregating .3,000 
board feet, and it is estimated that one-half of this \vill 
be lost before a second cutting is made, the loss, assuming 
a stumpage value of S3. .50 per thousand, is .S4.75. Thus 
it would be nuich less expensive to leave the trees than to 
]ilant. It often occurs, however, that the ground is 
already ])artially stocked, so that it may be necessary to 
actually plant only cme-half or one-fourth acre. If one-half 
acre is planted the cost will be $6, and if onlj^ one-fourth 
acre is planted the cost will be but $3. Thus the question 
must be decided for each area separately, considering the 
prol)al)le loss in timber and the cost of planting upon 
the ])asis of the actual area it will be necessary to plant. 
It should also be borne in mind that old trees suppress 
young growth in their immediate vicinity, so that after 
they are removed lilanks will mark their former position. 
Furthermore, a certain amount of damage to reproduc- 
tion will result from logging. Fortunately, the areas on 
which there is a great risk through deterioration of seed 
and shelter trees are not extensive. There are areas, 
hi>wever, on which satisfactory reproduction will not be 
secured even under the best of treatment, and on such 
areas it will he necessarj' to plant as a last resort. Further- 
more, even on areas where reproduction is generally good, 
we may expect occasional fail spots where it will be 
advantageous to plant. 

Further investigations on cut-over areas, with special 
reference to the effect of different degrees of cutting upon 
physical conditions which are now being conducted by 
the Fort Valley Forest Ex]-)eriment Station, will un- 
doubtedly determine with greater scientific certaintj' how 
our western yellow-pine forests should be managed. This 
study merely attem]:)ted to lay the scientific foundation 
upon which to base the broad principle of forest manage- 
ment of western yellow- pine in the Southwest. 



() 




000 922 983 8 $ 




Hollinger Corp. 
pH 8.5 



